Boronizing of steels has an advantage over carburizing or nitriding in being able to get harder surface of steels. Besides, boronizing is easier in treatments and does not require special equipments. Usually, as a main salt composition for immersion boronizing of steels, B4C, Na2B4O7·10H2O, Fe-B, KBF4, CaB6 or BN among others has been used. This paper describes the immersion boronizing which utilizes relatively inexpensive boric acid and potassium borate as major salt compositions and magnesium powder as a reducing agent. The results obtained were as follows: (1) A favorable boride layer was obtained by using magnesium powder as a reducing agent and boric acid and potassium borate as sources of boron supply. From the view point of economical efficiency in boronizing of steels, the best condition for boronizing was composed with H3BO3: 40 wt%, K2B4O7·5H2O: 20wt%, NaF: 15wt%, K2CO3: 15wt% and Mg powder: 10wt%. (2) In these experiments, the treatment for boronizing at 700°C did not yield any boride layer on steel, but the treatment above 750°C could yield boride layer. The satisfactory temperature for boronizing of steels was 900°C. (3) The composition of the boride layer was only Fe2B. (4) The microhardness of boride layer was from 1200 to 1800 for HV (0.1kg). (5) With increasing viscosity of salt, the thickness of boride layer tended to become more uneven. (6) The effect of the material of crucibles on the thickness of boride layer was not so noticeable for corundom and stainless steel crucibles. (7) In the case of carbon steels, the thickness of boride layer decreased slightly with increasing carbon content in steels.
This paper presents the deformation and fracture behaviors of stator windings subjected to bending moment. The experiments were carried out on four kinds of model windings, which were made with steel solid bar and copper strands and were impregnated with epoxy and polyester resins. The strain produced on the insulation surface and the deflection were measured under a uniform bending moment. The tangent delta characteristic was also measured and the occurrence of mechanical damages was discussed in relation to the variations in the tangent delta characteristic. The results obtained are as follows. (1) Stator windings impregnated with epoxy resin begin to break when the tensile strain on the insulation surface reaches to the ultimate strain of the insulation. (2) Stator windings impregnated with polyester resin are damaged on the compression side even at a very small strain level. Beyond a certain tensile strain, this failure accompanies the failure on the tension side. (3) The test method, by which the tangent delta characteristic is measured in addition to the measurement of strain and deformation, is effective for the study of the failure mechanisms of composite materials as well as stator windings. (4) It is considered that the use of an impregnating resin with higher mechanical strength is one of the useful means to get higher bending strength of stator windings.
This paper presents the deformation and fracture behaviors of the stator windings under repeated bending loads. In this investigation, two different kinds of stator windings, which were impregnated with epoxy and polyester resins, were examined. The stiffness of the stator windings was measured after a certain number of repeated bending loads was applied to the stator windings. At the same time, the tanδ characteristic was also measured and then the mechanical damage produced in the stator windings was discussed. The breakdown voltage was also measured in the last stage of each test and the fracture of the insulation was discussed. The results obtained are as follows. (1) When the repeated bending loads beyond a certain strain level were applied, a remarkable reduction was found in the stiffness of the stator windings with the increase of number of repeated bending loads. The stiffness at which the breakdown voltage decreased to the rated voltage was reduced to 60∼75% of the initial stiffness. (2) Below this cyclic strain level, the reductions in stiffness and breakdown voltage were comparatively small and no fatigue damage was found in the stator windings. (3) The maximum strain amplitude which could be applied to the stator windings without fatigue damage up to 107 cycles was about 30∼42% of the ultimate tensile strain.
When a spline shaft transmits a torque by meshing with the teeth of the boss, both torsional stress and flexural stress appear simultaneously at the foot of teeth. Therefore, the fatigue failure of a spline shaft can be considered as an example of the fatigue failure under combined flexural and torsional stresses. In view of the above fact, the author has proposed the equation for estimating the fatigue strength of a spline shaft, by using an expanded equation that incorporates size effect and mean stress into Gough's or Nishihara-Kawamoto's equation of endurance limit under combined flexural and torsional stresses. It is shown that the equation agrees satisfactorily with the experimental results. It is considered that the same idea might be applied to estimate the fatigue strength of other machine elements.
In order to clarify the effect of atmospheric-corrosion on fatigue fracture of metal, the fatigue test has been carried out on aluminum under reversed plane bending at the conditions of ambient pressure and 6×10-5 Torr. The difference of fatigue failure modes at these two environment was observed by optical and scanning electron microscopy, and the fatigue process during stress cycling of single crystal specimens was examined using the back reflection X-ray Laue method. The concentration of oxygen atoms adsorbed on the fracture surface was measured by using the X-ray micro-analyzer and the dark deposit on the fracture surface of the specimen tested in air was identified with the electron diffraction technique. The decrease of fatigue strength of aluminum in air was discussed from mechanochemical view points. The main results obtained are as follows: (1) The fatigue strength of alminum was increased by evacuating the air surrounding a specimen. The values of surface roughness and micro-Vickers hardness of the specimen tested in vacuum were higher than those in air. (2) The change of Laue asterism obtained on the single crystal specimen tested in vacuum was very intensive than that in air. (3) The concentration of oxygen atoms adsorbed on the fracture surface was very high on the specimen tested in air, but low in vacuum. (4) It was found from the electron diffraction analysis that the dark deposit on the fracture surface of specimen tested in air was the aggregate of fine particles of gamma-alumina having about 0.02∼0.05μm in diameter. No deposit was seen on the fracture surface of specimen tested in vacuum.
An attempt was made to clarify the effects of atmosphere (vacuum and vapor of benzene) and frequency on fatigue crack propagation of epoxy resins containing plasticizer, whose mechanical properties vary widely with the content of plasticizer involved in the specimen. The properties of fatigue crack growth in the specimen were investigated through the stress intensity factor K, in which the following relations between growth rate and K were assumed. dl/dN=A(Kmax)α1(ΔK)α2 and dl/dN=B(Kmean)β1(ΔK)β2, where A, α1, α2, B, β1 and β2 are constants respectively. The effect of frequency was investigated through the factor of (f)α3 and (f)β3 added to the above equations. The results obtained are summarized as follows: (1) The behavior of fatigue crack growth in the specimens in vacuum was similar to that in air. (2) The values of constants α1, α2, β1 and β2 obtained from the experiments in vapor of benzene are not so different compared with those in air. (3) The values of constants A and B obtained from the experiments in vapor of benzene are greater by one order than those in air for the specimens with low plasticizer contents, while they are comparable each other for the specimens with high plasticizer contents. (4) The values of constants α3 and β3 (which are negative) obtained from the experiments on the effect of frequency decrease as the content of plasticizer increases.
Thermal fatigue tests under in-phase condition were conducted on two typical types of steels, 0.16%C carbon steel (S15C) and austenitic steel (SUS304). Effects of in-phase and out-of-phase on fracture mode were examined. Characteristic features of microcracks under in-phase condition were found to be similar for both steels. The results obtained are summarized as follows: (1) Life reduction with in-phase thermal cycling (tension at high temperatures, compression at low temperatures) is remarkable when the maximum cycle temperature is high enough to produce creep strain and the strain range is fairy large. (2) Under in-phase condition, many grain boundary cracks initiate. They seem to reduce the ductility of materials more effectively than surface cracks. Only surface cracks are found under out- of-phase condition.
A study has been made of the oxidation of copper alloys containing rare earth elements at temperatures ranging from 500°C to 900°C. At temperatures below 800°C except at 500°C, the oxidation resistance in air was improved by the addition of rare earth elements, but it was hardly improved at 900°C. The time at which the oxidation process of copper began to proceed according to a parabolic law was delayed by the addition of rare earth elements at every investigated temperature. The activation energy for the oxide scale formation of copper was 34.7Kcal/mol. This value is approximately similar to the activation energy for the formation of cuprous oxide. The activation energies for the oxide scale formation of alloys containing rare earth elements were greater than that of copper. SEM photographs showed that there was a difference in appearance of surface oxide scale between copper and its alloys containing rare earth elements and that the mechanical pegging of oxide scales to matrix was noted in the alloys containing rare earth elements. Optical photographs of oxide scales also showed that the fine grained oxide scales which were formed on the matrix surface containing rare earth elements played an important role for the good adhesion of oxide scales to matrix.
As a part of the study of sintering behaviors of oxides, the thermal decomposition processes of La2(SO4)39H2O and La2(CO3)3 were investigated by using T. G. A. and X-ray diffraction. From the results obtained, the detailed mechanisms of their decomposition were inferred. (1) The thermo-gravimetric curves of the specimens were obtained in air, N2, and CO2 gas. The thermal decomposition curves showed three stages for La2(SO4)39H2O and two stages for La2(CO3)3. The chemical reaction of each stage was determined as follows. La2(SO4)3·9H2O: La2(SO4)3·9H2O→La2(SO4)3+9H2O (the first stage) La2(SO4)3→La2O2(SO4)+2SO2+O2 (the second stage) La2O2(SO4)→La2O3+SO3 (the third stage) La2(CO3)3: La(CO3)3→La2O3CO2+2CO2 (the first stage) La2O3CO2→La2O3+CO2 (the second stage) (2) These reactions were interpreted to be the first order reaction in all atmospheres. The rate constants for three decomposition stages in air of La2(SO4)39H2O were represented by k1=105.4exp(-13000/RT) (the first stage) k2=1012exp(-65000/RT) (the second stage) k3=1017exp(-134000/RT) (the third stage) and those for the two decomposition stages in air of La2(CO3)3 were k1=1013exp(-50000/RT) (the first stage) k2=1012exp(-67000/RT) (the second stage) (3) The lattice constants of La2O3 obtained were a=3.89Å and c=6.08Å. Lanthanum sulfate and carbonate decomposed to La2O3 at about 1500°C and 1000°C, respectively.